C-24 Stereochemistry of Marine Sterols: (22E)-24-Ethyl-24-methylchoesta-5,22-dien-3β-ol and 24-Ethyl-24-methylcholest-5-en-3β-ol

The C-24 configurations of (22E)-24-ethyl-24-methylcholesta-5,22-dien-3 beta-ol (1) and 24-ethyl-24-methylcholest-5-en-3 beta-ol (2), isolated from the Colombian Caribbean sponge Topsentia ophiraphidites, were determined to be R and S. respectively, by comparing their NMR data with those of stereodefined (24R)- and (24S)-samples that were synthesized in routes involving the orthoester Claisen rearrangement of Delta(23)-22-allylic alcohols. This is the first synthetic study where the Claisen rearrangement is used to introduce a C-24 quaternary center in a stereospecific manner with acceptable yield. X-ray analysis of 1 confirmed these stereochemical assignments.